This invention relates to wideband microwave attenuators and particularly to a wideband voltage controlled microwave attenuator employing semiconductor elements such as PIN diodes, and to coupling devices usable in such attenuators.
A circuit designer synthesizing low frequency circuits may employ devices designated resistor, capacitor, inductor without questioning these designations. As a circuit's operating range is extended into the megahertz region and into gigahertz, complex impedance characteristics of these elements become significant and must be considered. When a circuit is designed in the microwave region each circuit elements must be considered as a complex network. The task of constructing the circuit which can equivalently be represented by a resistance that remains substantially constant in value over a wide band of frequencies in the microwave region becomes a sophisticated problem.
Diodes have been arranged in mesh configurations as variable attenuators with d-c (direct current) bias applied to the diodes to vary their dynamic impedance for adjusting the attenuation levels. When such mesh circuits are operated at low frequencies, or even into the lower end of the microwave region, various reactive effects are minor and a two-port symmetrical attenuator can be operated with a characteristic impedance which is generally constant and equal to that of the transmission system. However, at higher microwave frequencies, phase changes along connecting lines become significantly large, as do reactive effects of the diodes. As the frequency becomes higher (e.g., to gigahertz), compensation for these phase-distorting effects becomes more important. One approach for such compensation involves the use of small dimensions for various parts and connecting lines. However, such small dimensions, in turn, lead to other construction problems. A discussion of these problems, as well as solutions thereof, is set forth in the paper, "The T-Pi Configuration in the Design of Extremely Broadband PIN Attenuators" by Hopfer et al., 1968 Proc. NEC 308-313, which is incorporated herein by reference.
D-C blocking capacitors may be interspersed between the mesh diodes to isolate the effect of bias currents being applied to one diode from affecting the other diodes. These blocking capacitors may be effective at lower frequencies, but at higher frequencies they do not behave as lumped capacitances and thus impair the operation of such circuits.